‘Celticity’ means the quality of being Celtic. ‘Celticization’ means the process or event(s) of becoming Celtic. Thus, Celticity involves a static or synchronic perspective and Celticization a dynamic, diachronic one. ‘Celtic’ is used here in a linguistic sense, because the debates of the past few decades over the term ‘Celtic’ seem to have left intact the concept of the Celtic languages as a proven and closely definable scientific fact, whereas Celtic culture (including Celtic art), Celtic identity, and so on, remain controversial and are prone to ambiguity (see e.g., James 1999; Sims-Williams 1998). Therefore, ‘Celtic’ here means belonging to the Indo-European sub-family of languages represented by the living Irish, Scottish Gaelic, Welsh, and Breton. These four, together with the recently extinct and now revived Manx and Cornish, and the ancient Celtiberian, Gaulish, Galatian, and Lepontic together form a genetic language family. That means, rather than having anything to do with biological genetics per se, that these languages show systematic similarities—more closely with one another than with any other attested language or group of languages—implying that they descend from a single proto-language, usually called ‘Common Celtic’ or ‘Proto-Celtic’, which had been the speech of a people, who had once formed a coherent community, occupying a particular geographic territory, at a particular time. The principle is the same as Latin and the Romance languages (French, Italian, Portuguese, Romanian, and Spanish), but in the case of classical Latin and its daughter languages the ancient proto-language is fully attested and its epicentre can be pinpointed in time and space. It is highly unlikely that Celtic or a language directly ancestral to it was the first language spoken by human beings in any part of Europe. For example, Celtic was not the language of Palaeolithic France nor of Mesolithic Ireland. Proto-Celtic is the descendant of another reconstructable language, Indo- European, which itself dates, according to various experts, somewhere within the Neolithic or Early Bronze Age (Mallory 1989). How does a language appear in a country? We shall consider three general paradigms.